Kevin Cooper, the death row inmate convicted of killing a Chino Hills family in 1985, once again was denied a request for additional DNA testing.

Cooper's attorneys recently filed a suit in federal district court challenging a state court's denial of his request to obtain additional DNA testing pursuant to a state statute, according to the opinion from the U.S. 9th Circuit Court of Appeals.

Cooper, 54, believes additional testing of blood samples from the crime scene may prove he's not guilty of killing four family members.

"In the complaint, Cooper alleges that he is the target of a long-running conspiracy, involving members of the San Bernardino County Sheriff's Department and others, to manipulate evidence and prevent him from proving that he was framed," according to the opinion, which was filed Thursday.

Cooper has been on death row since he was convicted of four counts of first-degree murder in 1985.

"Since then his case has traveled up, down, and around the federal and state judiciaries," according to the opinion.

At one time, Cooper was within a few hours of execution at San Quentin prison but it was postponed due to last-minute appeals.

Officials involved in the case are frustrated with the delays.

"This has been an abuse of the appellate process and is exactly why this part of capitol litigation must be fixed," said San Bernardino County District Attorney Michael Ramos, who is one of several public officials named in Cooper's complaint.

"Justice for the victims' families is long overdue."

In 2011, a judge turned down Cooper's request to allow further DNA testing.

Cooper was convicted and sentenced to death almost 28 years ago for the hatchet slayings on June 4, 1983, of a married couple, their daughter and a houseguest.

Cooper has long maintained his innocence, and he pushed for DNA testing that the state's appellate lawyers agreed in 2001 to allow.

Cooper hoped the testing would show that some of the blood found at the crime scene came from people other than Cooper and those killed.

Instead, Cooper's DNA was found in the blood samples recovered from the crime scene. That led to accusations from Cooper and his attorneys that the evidence was planted.

Last year, San Diego County Superior Court Judge Kenneth K. So rejected a request from Cooper's attorneys to conduct further DNA testing.

In his 29-page order, So repeated the evidence of Cooper's guilt - evidence that So said was bolstered by previous DNA testing.

"After the advent of DNA testing and after the post-conviction testing that was conducted, it was established that (Cooper's) DNA and the victims' DNA were on the same article of clothing," So wrote.

"This very powerful incriminating evidence is consistent with the other overwhelming evidence of (Cooper's) guilt introduced at his trial."

Two days before the slayings, Cooper escaped from the California Institution for Men in Chino, where he was imprisoned for committing two residential burglaries in Los Angeles County.

It was the 12th time Cooper had escaped from a jail or prison, So wrote in his order.

Cooper acknowledges that after his escape, he took refuge in a vacant home on a property adjacent to the lot where there killings took place. But he denies responsibility for the killings.

Doug and Peggy Ryen, their 10-year-old daughter Jessica Ryen, and 11-year-old houseguest Christopher Hughes, died after being attacked with a hatchet, ice pick and knife. The Ryens' 8-year-old son, Joshua Ryen, survived despite being badly injured in the attack.

An execution date is pending for Cooper or any other California death row inmates because of judicial concerns that the state's lethal injection method is inhumane.

"Until litigation surrounding the lethal injection procedure is completed all executions in California are on hold," said Chris Lee, spokesman for the District Attorney's Office.

Recently, Chief Justice Tani Cantil-Sakauye said she did not expect executions in California to resume for at least three years because of problems with the lethal injection process, according to the Death Penalty Information Center website.